The present invention relates to vacuum gauges for registering the subatmospheric pressure of a gas. More particularly, the present invention relates to a vacuum gauge in which the subatmospheric pressure of the gas is registered as a function of the reduced thermal conductivity of the gas at the subatmospheric pressure to be registered.
The prior art has provided thermal conductivity vacuum gauges such as the well known Pirani gauge. Pirani gauges employ two load resistors having positive temperature coefficients connected in the outlying legs of a half bridge circuit. The half bridge circuit also employs a power supply to apply constant voltages, at a constant current, across the two load resistors and a voltmeter to register voltage imbalances between the outlying legs of the half bridge circuit.
One of the two load resistors is supported in an insulated manner such that the heat produced from its electrical power output is by in large conducted in the gas. As the gas pressure falls below atmospheric pressure, there will be less of a statistical concentration of gas molecules to conduct the heat. As a result, the resistor will self-heat to a temperature above ambient temperature. The elevated temperature will increase the resistance of the resistor and therefore, the voltage drop produced by the resistor. The voltage drop of the resistor has two additive components, namely, ambient temperature and the temperature rise above ambient temperature as a function of the reduced thermal conductivity of the gas at subatmospheric pressures. Since the thermal conductivity of the gas is a function of its subatmospheric pressure, the latter component of the voltage drop is also a function of subatmospheric pressure. The other load resistor is connected to a heat sink at ambient temperature to produce a voltage drop related to ambient temperature. The half bridge circuit acts to subtract the voltage drops produced by the load resistors; and the resultant voltage, which is essentially a function of subatmospheric pressure, is read from the voltmeter and correlated with subatmospheric pressure.
One major problem with Pirani gauges and in general, vacuum gauges using resistors, is that their precision of measurement is completely dependent upon the resistors having identical temperature characteristics and the degree to which constant and equal voltages can be precisely applied by the power supply across the resistors. Power supplies and resistors having the requisite precision are expensive. Other problems with Pirani gauges and the like relate to the operating temperature of their resistors which can exceed 340 K. At such temperatures, the resistors begin to radiate heat to introduce an inaccuracy in the readings. Furthermore, since pump oils used in vacuum pumps begin to break down at such temperatures, contamination can be introduced into vacuum pumped environments by the use of such gauges in the monitoring of the function of vacuum pumps. A further disadvantage is that such gauges have a relatively long response time until reaching a steady state at which pressure readings may be taken. As will be discussed, the present invention overcomes these and other drawbacks of such prior art vacuum gauges.